Electric motors can be characterized by a torque constant, Kt, which is basically the torque produced by the motor divided by the coil current required to produce that torque. Motors with a high torque constant generally are useful for producing high torque at low RPM; whereas motors with a lower torque constant can be more suited to producing higher speeds, where high torque is not required. In a very general sense, torque constant is a measure of the electromagnetic coupling between the coil assembly and the magnetic rotor assembly. Higher coupling produces a higher torque constant as compared to lower coupling. But one of the downsides of using a motor with a higher torque constant is the relatively higher back EMF and induced eddy currents that are generated at the higher speeds. That back EMF and the induced eddy currents lower the maximum speed that is achievable by the motor and result in higher losses at the higher speeds, i.e., less efficient operation. Therefore, it might be desirable to vary the torque constant over the range of operation, providing a higher torque constant at lower speeds and a lower torque constant at higher speeds.